1. Field of the Invention
The present invention relates to a photoelectric conversion device and image capturing device.
2. Description of the Related Art
Recently, solid-state image capturing devices using amplifier type photoelectric conversion devices, especially MOS photoelectric conversion devices have attracted attention. In the MOS solid-state image capturing device, for example, when the sun falls within the angle of view and very strong light enters the image capturing plane, a portion of the image receiving strong light is captured as a dark portion. This phenomenon is called high-brightness darkening, darkening, or blackening phenomenon. This phenomenon will be referred to as “darkening”.
Darkening occurs in a CDS (Correlated Double Sampling) method of reading out an S output (data level) and N output (noise level) from a pixel, calculating their difference, and outputting it. If high-brightness light is incident in reading out an N output, charges generated by light enter the floating diffusion, decreasing the voltage of the floating diffusion. In response to this, an N output from a pixel drops. If the output drops excessively, the difference between the S and N outputs becomes very small, causing darkening.
Japanese Patent Laid-Open No. 2004-222273 discloses an arrangement for reducing darkening. FIG. 2 is a circuit diagram showing the arrangement of a pixel having a function of suppressing darkening. A pixel unit 101 may include a photoelectric converter 1, a transfer gate 2, a charge-voltage converter 5, an amplifying transistor 4, and a reset transistor 3. The photoelectric converter 1 includes, for example, a photodiode and generates charges upon receiving light. The transfer gate 2 transfers charges generated by the photoelectric converter 1 to the charge-voltage converter 5 in accordance with a transfer pulse. In general, the charge-voltage converter 5 may be formed as a floating diffusion. The amount of charges transferred to the charge-voltage converter 5 determines its voltage. The amplifying transistor 4 amplifies the voltage of the charge-voltage converter 5, and outputs the amplified voltage onto a pixel output line 130. The reset transistor 3 resets the voltage of the charge-voltage converter 5 to a predetermined voltage. The source of the reset transistor 3 is connected to the charge-voltage converter 5 and the gate of the amplifying transistor 4. The drain of the reset transistor 3 is set to a predetermined voltage together with that of the amplifying transistor 4.
In reading out the noise level, a clipping transistor 6 receives at its gate the voltage Vclip which determines the clip level. Letting Vt be the threshold of the clipping transistor 6, the voltage of the pixel output line 130 in reading out the noise level is so restricted as to not become lower than Vclip−Vt. Thus, a sufficient difference that is sufficient to cope with an optical signal is obtained between the data level and the noise level, suppressing darkening.
As another method, Japanese Patent Laid-Open No. 2001-024949 discloses an arrangement having a correction means for correcting the differential process between the signal level and the noise level in accordance with an output from an image capturing condition detection means when performing the differential process.
According to the technique as shown in FIG. 2, it is difficult to set the voltage Vclip applied to the gate of the clipping transistor 6. When a normal noise level is output, the clipping transistor 6 must be completely OFF so as to prevent the clipping transistor 6 from affecting the voltage of the pixel output line 130. This is because, if the clipping transistor 6 is ON in reading out the noise level, the noise level varies and no accurate noise level can be subtracted from the data level.
Considering manufacturing variations in the clipping transistor 6, for example, Vclip must be set lower by about 0.2 V than the threshold of the clipping transistor 6 in order to completely turn off the clipping transistor 6. This means that, if the voltage of the pixel output line 130 drops owing to charges flowing into the charge-voltage converter 5, the clipping transistor 6 is not turned on unless the voltage becomes lower by 0.2 V or more than the threshold of the clipping transistor 6.
When the photoelectric converter 1 holds sufficient charges and the voltage appearing on the pixel output line 130 exhibits large amplitude, the fluctuation of about 0.2 V in the reset level is negligibly small.
However, when the sensitivity of the photoelectric converter 1 decreases due to downsizing of the pixel or the quantity of incident light is small in recording a dark object, the amount of charges stored in the photoelectric converter 1 is small, and the voltage appearing on the pixel output line 130 exhibits small amplitude. In this case, the voltage fluctuation of 0.2 V is not negligible.
In this case, the fluctuation of the voltage Vclip must be suppressed within, for example, 0.05. However, since the threshold varies by about 0.1 V owing to manufacturing variations in the clipping transistor 6, suppressing the fluctuation of the voltage Vclip is insignificant.